This invention relates to a semiconductor apparatus containing at least a bipolar transistor and more particularly to a semiconductor apparatus having a sufficiently great current gain at a low temperature, too, and capable of a high speed operation.
Hereinafter, a description will be given about an npn bipolar transistor by way of example. FIGS. 2(A) and (B) of the accompanying drawings are a sectional view of a conventional npn bipolar transistor and an impurity distribution along line a - a' shown on pages 80 and 81 of "Very High Speed Bipolar Devices" (published by Baifukan, 1985). In the drawings, reference numeral 1 represents an n-type emitter region; 2 is a p-type intrinsic base region; 3 is a p-type extrinsic base region; 4 is an n-type low concentration collector region; 5 is an n-type high concentration collector region; 6 is a thick SiO.sub.2 film for isolation; 8 is a p-type Si substrate; and 10 is a high concentration p-type region as a channel stopper.
Here, the intrinsic base region 2 represents a portion of the p-type base regions 2, 3 functioning as a path through which the electrons injected from the emitter flow out to the collector and the extrinsic base region 3 represents a portion of the p-type base region other than the intrinsic base region 2.
In FIG. 2(B), reference numeral 12 represents the concentration distribution of an n-type impurity and 13 represents the concentration distribution of a p-type impurity. As shown in the diagram, the conventional bipolar transistor uses an impurity region having a high concentration of higher than 1.times.10.sup.20 /cm.sup.3 as the emitter 1. The intrinsic base region 2 is composed of a p-type region having a lower impurity concentration than the emitter 1. As illustrated in this example the value is set ordinarily to 1.times.10.sup.18 /cm.sup.3 or below.
The conventional bipolar transistor described above operates normally at room temperature but involves the problem in that its current gain drops remarkably at a low temperature of 200.degree. K. or below FIG. 3 shows the temperature dependence of the common-emitter current gain according to an actual measurement. Line l.sub.1 represents the temperature dependence of the current gain of the conventional structure shown in FIG. 2. The current gain which is about 150 at room temperature drops to 1 or below at 100.degree. K.
FIG. 4(A) is a sectional view of the bipolar transistor discussed by H. Yagi et al. in "Technical Digest 1974 International Electron Devices Meeting", p.p. 262-265. FIG. 4(B) shows the impurity distribution on the section b - b'. As discussed in this reference, the current gain generally becomes smaller with a lower emitter concentration. However, if the thickness of the low concentration emitter region 9 is smaller than the diffusion length of a positive hole, a sufficiently large current gain can be observed at room temperature even in such a structure wherein the impurity concentration of the emitter region is lower than that of the intrinsic base region. However, this structure, too, involves the problem that the current gain drops remarkably at low temperature in the same way as in the structure shown in FIG. 2. Line l.sub.2 in FIG. 3 represents the actually measured value of the current gain of the bipolar transistor having the structure shown in FIG. 4. Whereas the current gain is about 100 at room temperature, it drops to about 4 at 90.degree. K.
In order to avoid the remarkable drop of the current gain of the bipolar transistor in the low temperature operation, IEEE, Trans Electron Devices, ED-34 (1987), pp. 139-142 discusses a method which sets the emitter concentration to 1.times.10.sup.19 /cm.sup.3 or below. However, this reference does not at all disclose a definite structure. When the inventors of the present invention actually measured a bipolar transistor shown in FIG. 5 and having an emitter concentration of 5.times.10.sup.18 /cm.sup.3 and a base concentration of 1.times.10.sup.18 /cm.sup.3, it was found that a sufficiently large current gain could also not be obtained at low temperature in accordance with this structure. This result is represented by line l.sub.3 in FIG. 3. When compared with the structures shown in FIGS. 2 and 4(A), only a current gain of as small as 10 or below could be obtained at low temperature of 100.degree. K. or below, though temperature dependence of the current gain dropped.
The bipolar transistor structure in accordance with another prior art technique is discussed in IEEE, Trans. Electron Devices, ED-27 (1987), pp. 563-570. It is a bipolar transistor having a high impurity concentration intrinsic base region. According to the result described in this reference, however, the current gain which is 150 at room temperature drops to 16 to 30 at 77.degree. K. in a device having a maximum base concentration of 2.5.times.10.sup.18 /cm.sup.3. Since both the emitter region and base region have a high concentration impurity in the structure of this reference, the device involves a problem in that its emitter-base junction breakdown voltage is low.
Still another method of improving the current gain in the low temperature operation is described in Appl. Phys Lett., Vol 45 (1984), pp. 1086-1088. From this reference a method of forming a heterojunction between the emitter and base by using AlGaAs for the emitter and GaAs for the base is well known in the art. However, the formation of such a heterojunction is not only difficult from the aspect of production technique but is also time-consuming and expensive to produce. Since different kinds of materials are bonded between the emitter and the base, the device is not free from the problem that recombination of carriers is likely to occur on the interface.
On the other hand, the reason why the bipolar transistor exhibits only an extremely low current gain at low temperature in accordance with the prior art technique is explained in IEEE, Trans. Electron Devices, ED-15 (1968), pp. 732-735. Namely, since the impurity of the emitter 1 contains an impurity having a high impurity concentration of 1.times.10.sup.19 /cm.sup.3 or more, the bandgap of the emitter region is smaller than that of the intrinsic base region. This will be explained elsewhere in further detail.
On the other hand, "Very High Speed Compound Semiconductor Devices" (published by Baifukan, 1986) states on page 108 as follows:
"HBT having a small base-emitter capacitance can be accomplished by setting the relation N.sub.E .ltoreq.10.sup.18 /cm.sup.-3, P.sub.B .gtoreq.10.sup.19 /cm.sup.-3, which is impossible by the homo-junction, to reduce the base resistance." PA0 "HBT having a small base-emitter capacitance can be accomplished by setting the relation N.sub.E .ltoreq.10.sup.18 /cm.sup.3, P.sub.B .gtoreq.10.sup.19 cm.sup.-3, which is impossible by the homo-junction, to reduce the base resistance."
(Here, N.sub.E and P.sub.B represent the impurity concentrations of the emitter and the base, respectively, and HBT represents a hetero-junction bipolar transistor.) As can be understood from this statement, it has been believed conventionally that if the transistor having the homo-junction (junction by the same material) between its emitter and base such as the transistor of the present invention has a concentration distribution such as the distribution of the present invention, the transistor has low injection efficiency at normal temperature and a low current gain and is therefore not much practical.
On the other hand, Japanese Patent Laid-Open No. 190758/1987 proposes a homo-junction type bipolar transistor causing a bandgap difference between the emitter region and the base region by making the base impurity concentration incomparably greater than the emitter impurity concentration, that is, by setting the base impurity concentration to at least 2.times.10.sup.20 /cm.sup.3, in order to avoid the drop of the current gain, to reduce the base resistance and thus to improve the operation speed.
However, the bandgap narrowing value .DELTA.Eg of the silicon semiconductor by the high impurity concentration N.sub.A disclosed in the reference described above [i.e. .DELTA.Eg=22.5 (N.sub.A /10.sup.18).sup.1/2 ; (meV)] is the value at room temperature (approx. 300.degree. K.) as can be understood from the formula proposed by P. D. Lanyon et al. in Technical Digest 1978 International Electron Devices Meeting, pp. 316-319.
However, this reference does not mention the problem of the low temperature operation below 200.degree. K. of the homo-junction bipolar transistor. Moreover, the bandgap narrowing value .DELTA.Eg is much greaxer than the value that is commonly believed at present (refer to the formula on page 27 and FIG. 9, S. E. Swirhun et al., Technical Digest 1986 International Electron Devices Meeting, pp. 24-27) and as a result, the common-emitter current gain of the bipolar transistor is also an overestimated value. As a result of studies, the inventors of the present invention found that if the value which was commonly believed was employed, the common-emitter current gain having a sufficiently large value could not be obtained.
On the other hand, since the impurity concentration of the intrinsic base region in the reference described above is an extremely large value of above 2.times.10.sup.20 /cm.sup.3, it was found as a result by studies of the present inventors that the reference did not consider the effect of remarkable drop of the common-emitter current gain due to Auger recombination.